The present invention relates to improvements in disc drive assemblies, and more particularly, the invention relates to an improved base plate for mounting the head/gimbal assembly of a magnetic disc drive to an actuator arm.
In disc drives, the head/gimbal assembly is attached to the actuator arm by swaging a base plate to the actuator arm. The base plate holds the head/gimbal assembly in place and restricts relative movement during subsequent assembly, transportation, shipping and operation of the disc drive. Typically, both the base plate and the actuator arm are constructed of metal materials. For example, a stainless steel base plate may be swaged to an extruded aluminum actuator arm.
In order to swage the base plate into the actuator arm, a tubular boss of the base plate is fit into an opening in the actuator arm. A ball is then driven through the tubular boss. Because the boss has an inner diameter smaller than the outer diameter of the ball, the ball exerts a deforming load on the boss. The boss plastically deforms to press against the opening walls in the actuator arm as the ball passes through the boss. To hold the head/gimbal assembly in place, the interface between the base plate and the actuator arm must develop a suitable minimum retention torque, preferably at least about ten inch-ounces. The amount of retention torque in a swage connection is dictated by the frictional force between the swaged members, which in turn is based on the friction between the boss material and the actuator arm material, the residual stresses in the boss of the swage, and the elastic stress in the actuator arm.
The frictional force developed between two surfaces depends on the coefficient of friction between the two surfaces, and the force perpendicular to the plane of relative motion between the two surfaces, or normal force. Thus, the friction between the outer surface of the boss and the inner surface of the opening in the actuator arm is dependant on the force exerted perpendicular to the interface (the normal force) between the boss and the actuator arm and the coefficient of friction between the two surfaces.
In the past the materials used to construct the actuator arm and the base plate were metal, such as aluminum and steel. In a swage connection, both the boss and the arm deformed during swaging, smoothing both metal surfaces, and creating a somewhat smooth interface between the metals. Neither the boss nor the arm returned to its original dimensions after the deforming load was released. As a result, no force is necessary to maintain the boss""and arm""s newly deformed dimensions, once they have been deformed by the ball. The resulting smooth interface of the swage connection diminished the coefficient of friction between the boss and the actuator arm, a typical coefficient of friction between a stainless steel base plate and aluminum actuator arm that are swaged together is in the range of 0.1 to 0.3.
In one embodiment of the invention, the base plate comprises a plurality of portions defining a cylinder having an outer surface with a relaxed outer diameter greater than the inner diameter of the opening in the actuator arm. The boss is insertable into the opening in the actuator arm. As the boss engages the opening in the actuator arm the plurality of portions elastically deform to conform to and bias against the inner surface of the actuator arm. In one form of this second embodiment, the actuator arm and base plate are constructed of polymer material having a yield stress in the range 4 to 9 Kpsi and a modulus of elasticity in the range 145 to 450 Kpsi.
Another aspect of the present invention is a method for securing the head/gimbal assembly of a disc drive to an actuator arm with a base plate. The base plate boss has a plurality of portions defining a cylinder having an outer diameter greater than the inner diameter of the opening in the actuator arm. The head assembly is sandwiched between the base plate and the actuator arm so that the raised portions of the base plate is aligned with the opening of the head assembly. The raised cylinder is then pressed into the opening in the actuator arm so that the plurality of portions elastically deform to conform to and bias against the inner surface of the opening in the actuator arm.
The present invention overcomes the limitations inherent in swaging the base plate to the actuator arm. The boss is elastically deformed to fit into the opening in the actuator arm, permitting the boss to return to its original shape and dimensions when the deforming load is released. Because the opening in the actuator arm continuously applies the deforming load on the boss, a significant normal force is created between the two surfaces. As a result, a high frictional force, and therefore a high retention torque, is created between the boss and the actuator arm.